The convenience associated with wireless charging (or “contactless charging”) of portable electronic devices has evoked extensive interest in the development of this technological area. In general, wireless charging relies on near field magnetic induction between coils. More specifically, in a system that implements wireless charging, when a power receiving device (e.g., a cellular telephone) is brought into close proximity to a power transmitting device (e.g., a wireless charging pad, table, or other surface), power is transmitted from the power transmitting device to the power receiving device through magnetic inductive coupling between coils in the devices (i.e., between the “primary coil” in the power transmitting device and a “secondary coil” in the power receiving device). An alternating current is passed through the primary coil of the power transmitting device, and that alternating current generates a time-varying magnetic field around the primary coil. When the time-varying magnetic field impinges on the secondary coil in the power receiving device, a voltage is induced in the secondary coil. Additional circuitry within the power receiving device produces an output voltage that can be used to charge a battery or otherwise power the power receiving device.
The Qi interface standards developed by the Wireless Power Consortium set forth various wireless charging specifications to promote interoperability across rechargeable electronic devices. For example, among other things, the Qi interface standard sets forth a communication protocol for a power receiving device to communicate with a power transmitting device in order to request more or less power. More particularly, the power receiving device communicates packets of control data to the power transmitting device, which include control error packets, received power packets, charge status packets, and end power transfer packets, among other possible packets. Based on the information contained within a received packet, the power transmitting device may adjust its operating point (e.g., increase or decrease the primary coil current to increase or decrease the oscillating magnetic flux enclosed by the primary coil(s), referred to as the “power signal”).
The power receiving device communicates to the power transmitting device using backscatter modulation. More specifically, the power receiving device modulates the amount of power that it draws from the magnetic field impinging upon the secondary coil. The power transmitting device detects this as a modulation of the current through and/or voltage across the primary coil. In other words, the power receiving device and the power transmitting device use an amplitude modulated power signal to provide a power receiving device-to-power transmitting device communications channel.
Implementing wireless charging in accordance with the Qi interface standard is not suitable for all types of portable electronic devices. Essentially, the power receiving device must have sufficient physical size and processing power to perform the processing and modulation associated with determining a received power level and communicating with the power transmitting device using the Qi packet-based communication protocol. Accordingly, application of the Qi interface standard or other wireless charging techniques tends to be limited to systems that include relatively large or complex power receiving devices.